Ethernet/802.3 Frames

Preamble

7 bytes of 0101010101... This is used to synchronise the receiver.
 

Start Of Frame

1 byte, thus: 01010111.
 

Source and Destination Address

each 6 bytes (48 bits!), and are uniquely assigned by IEEE. This is called a station's MAC address (or MAC-level address). All stations on a segment examine the destination address of all frames to see if it was addressed to them.
 

Type field

indicates which higher-level protocol created this frame, eg 0x0800 for IP. In 802.3 (rarely used to carry IP packets) this field gives the length (in bytes) of the data field.
 

Data field

between 46 and 1500 bytes of data. NB: minimum frame size is thus 64 bytes. The data field usually contains an IP datagram.